This invention relates to a circuit for controlling the power to at least two loads, in a manner so as to minimize interference with the AC power supply. The circuit comprises a power semiconductor unit, switching elements and a control unit for influencing the power consumption of the loads.
The control as well as the regulation of lamps, in particular halogen lamps, whose cold resistance is very low and whose starting and switching currents are very large, gives rise to problems because switching of the lamps gives rise to disturbances on the AC power-supply mains. For power control of such lamps with step control of the power consumption, it is known to switch up and down between the main power modes, for example, parallel operation, series operation and the off-state of two lamps, or to turn on the main power modes only for specific time intervals. When the loads are turned on and off, undesirable switching surges are produced, which cause corresponding mains-voltage fluctuations.
The human eye is particularly sensitive to such mains voltage fluctuations in a frequency range around 10 Hz. Therefore, for the relevant equipment there are regulations prescribing sufficiently small switching transients, such as the IEC standard 555 "Disturbances in supply systems caused by household appliances and similar electrical equipment". This standard in principle defines the harmonic content above 50 Hz and the mains voltage fluctuations below 50 Hz which are permissible to avoid undesirable flicker. In concrete terms this means that power control with short switching times between the main power modes is no longer possible for high loads, for example 1000 W, because the resulting mains voltage fluctuations are then no longer permissible.
Thus, high-power lamps for example cannot automatically be set to full power directly in series or parallel operation. Moreover, in order to reduce the power below the power at the supply voltage and in the case of series operation of the lamps, the supply voltage has to be reduced. It is customary to use switched-mode power supplies for this purpose but these are very complex and expensive. Another possibility is to suppress individual phases of the supply voltage, but this may also lead to substantial mains disturbances.
German Offenlegungsschrift 37 26 535, which corresponds to U.S. Pat. No. 4,871,961 (Oct. 3, 1989) and which is hereby incorporated by reference, describes a method for controlling the power of electrical loads with minimal switching transients, employing an intricate circuit with three triacs or six thyristors. For example in the case of two loads this circuit allows control with different switching modes, i.e. control of the two loads individually, in parallel or in series. This circuit requires a complex and therefore expensive electronic control device for the multitude of power semiconductors used and is susceptible to self-triggering and hence damaging of the power semiconductors.